Pleated aligned web filter

ABSTRACT

A filter element in the form of a nonwoven self-supporting filtration web having rows of folded or corrugated spaced-apart pleats, the web containing continuous thermoplastic fibers a majority of which are aligned at 90°±20° with respect to the row direction. The filter element can be made by forming rows of pleats in such a nonwoven web and cutting the web to a desired size and shape. The filter elements can provide improved mechanical and filtration properties and can exhibit reduced susceptibility to pleat deformation and the loss of space between pleats.

CROSS-REFERENCE TO RELATED APPLICATION

This application is a divisional of application Ser. No. 11/457,933,filed Jul. 17, 2006, now issued as U.S. Pat. No. 7,622,063, which is acontinuation-in-part of application Ser. No. 10/818,091 filed Apr. 5,2004, now abandoned, the entire disclosures of which are incorporatedherein by reference.

This invention pertains to pleated filters that have fiber alignment inthe direction of web formation.

BACKGROUND

Meltblown nonwoven fibrous webs are used for a variety of purposesincluding filtration (e.g., flat web and pleated filters), insulation,padding and textile substitutes. Patents or patent applications relatingto meltblown nonwoven fibrous webs include U.S. Pat. Nos. 3,959,421(Weber et al.), 4,622,259 (McAmish et al.), 5,075,068 (Milligan et al.),5,141,699 (Meyer et al.), 5,273,565 (Milligan), 5,405,559 (Shambaugh),5,523,033 (Shambaugh et al.), 5,652,048 (Haynes et al.), 5,665,278(Allen et al.), 5,667,749 (Lau et al.), 5,695,487 (Cohen et al.),5,772,948 (Chenoweth) and 5,811,178 (Adam et al.), and Published PCTApplication No. WO 95/03114 (University of Tennessee ResearchCorporation). Patents or patent applications relating to filters includeU.S. Pat. Nos. 3,780,872 (Pall), 4,547,950 (Thompson), 5,240,479(Bachinski), 5,709,735 (Midkiff et al.), 5,820,645 (Murphy, Jr.),6,165,244 (Choi), 6,521,011 B1 (Sundet et al. '011) and D449,100 S(Sundet et al. 100), U.S. Patent Application Publication Nos. US2003/0089090 A1 (Sundet et al. '090) and US 2003/0089091 A1 (Sundet etal. '091), and European Published Application No. 1 437 167 A1 (MorimuraKKK).

SUMMARY OF THE INVENTION

Nonwoven web manufacture typically involves deposition of fibers on amoving collector surface. Perhaps partly as a consequence of thismotion, the collected web may exhibit a small degree of fiber alignmentin the machine direction, and to a small extent some anisotropicphysical properties (e.g., tensile strength) in the machine andtransverse directions. Nonwoven web manufacturers often strive howeverto make products having well-balanced and generally isotropic physicalproperties.

We have found that by forming nonwoven webs having much greater thannormal fiber alignment in the machine direction and forming theresulting webs into pleated filtration media having spaced-apart pleatsgenerally transverse to the machine direction, we can obtain filtershaving reduced pleat deformation at high filter flow rates.

The present invention provides, in one aspect, a filter element thatcomprises a nonwoven self-supporting filtration web having rows offolded or corrugated spaced-apart pleats and that contains continuousthermoplastic fibers a majority of which are aligned at 90°±20° withrespect to the row direction.

The invention provides, in another aspect, a method of making a pleatedfilter element, which method comprises:

forming rows of spaced-apart pleats in a nonwoven filtration web thatcomprises continuous thermoplastic fibers a majority of which arealigned at 90°±20° with respect to the row direction; and

cutting the pleated web to a desired size and shape to form aself-supporting pleated filter element.

These and other aspects of the invention will be apparent from thedetailed description below. In no event, however, should the abovesummaries be construed as limitations on the claimed subject matter,which subject matter is defined solely by the attached claims, as may beamended during prosecution.

BRIEF DESCRIPTION OF THE DRAWING

FIG. 1 is a schematic side view of a meltblowing apparatus for makingnonwoven webs having fibers substantially aligned in the machinedirection.

FIG. 2 is an overhead view of a vacuum collector for use in theapparatus of FIG. 1.

FIGS. 3 a-6 b are radar plots showing fiber alignment.

FIG. 7 is a perspective view of pleated filtration media.

FIG. 8 is a perspective view, partially in section, of a pleated filtermounted in a frame.

FIG. 9 is a graph showing filter pressure drop vs. air velocity.

FIG. 10 is a schematic illustration of an apparatus for making thedisclosed pleated filters.

Like reference symbols in the various figures of the drawing indicatelike elements. The elements in the drawing are not to scale.

DETAILED DESCRIPTION

The term “nonwoven web” means a fibrous web characterized byentanglement or point bonding of the fibers.

The term “filtration web” means a porous web capable of removing atleast particles having an average particle diameter greater than 10 μmfrom a stream of air flowing at a 0.5 m/sec face velocity at an initialpressure drop no greater than about 50 mm H₂O.

The term “size” when used with respect to a fiber means the fiberdiameter for a fiber having a circular cross section, or to the lengthof the longest cross-sectional chord that may be constructed across afiber having a non-circular cross-section.

The term “continuous” when used with respect to a fiber or collection offibers means fibers having an essentially infinite aspect ratio (viz., aratio of length to size of e.g., at least about 10,000 or more).

The term “Effective Fiber Diameter” when used with respect to acollection of fibers means the value determined according to the methodset forth in Davies, C. N., “The Separation of Airborne Dust andParticles”, Institution of Mechanical Engineers, London, Proceedings 1B,1952 for a web of fibers of any cross-sectional shape be it circular ornon-circular.

The term “attenuating the filaments into fibers” means the conversion ofa segment of a filament into a segment of greater length and smallersize.

The word “meltblowing” means a method for forming a nonwoven web byextruding a fiber-forming material through a plurality of orifices toform filaments while contacting the filaments with air or otherattenuating fluid to attenuate the filaments into fibers and thereaftercollecting a layer of the attenuated fibers.

The term “meltblown web” means a nonwoven web made using meltblowing.

The term “nonwoven die” means a die for use in meltblowing.

The terms “meltblown fibers” and “blown microfibers” mean fibers madeusing meltblowing.

The term “machine direction” when used with respect to a meltblown webor to a meltblowing apparatus for meltblown web formation means thein-plane direction of web fabrication.

The term “transverse direction” when used with respect to a meltblowingapparatus or a meltblown web means the in-plane direction perpendicularto the machine direction.

The term “row direction” when used with respect to a pleated filterelement means a direction generally parallel to the pleat ridges andvalleys in a filter element having a folded structure with parallel,generally sharp-edged creases, and to a direction generally parallel tothe pleat crowns and base regions in a filter element having acorrugated structure with parallel, generally smooth undulations.

The term “spaced-apart” when used with respect to a pleated filterelement made from a folded or corrugated web means that there issufficient distance between neighboring pleats so that flow through thefilter element is generally transverse to the web.

The term “self-supporting” when used with respect to a web means a webhaving sufficient coherency and strength so as to be drapable andhandleable without substantial tearing or rupture, and when used withrespect to a pleated filter means a filter whose pleats have sufficientstiffness so that they do not collapse or bow excessively when subjectedto the air pressure typically encountered in forced air ventilationsystems.

A variety of polymers may be employed to make the disclosed alignedfiber webs. Representative polymers are thermoplastic, extrudable andcan be processed using a meltblowing apparatus, and include polyolefinssuch as polyethylene, polypropylene or polybutylene; polyamides;polyesters such as polyethylene terephthalate; and other materials thatwill be familiar to those skilled in the art. Polyolefins areparticularly preferred.

A variety of sorbent particles can be added to the nonwoven webs ifdesired. Representative sorbent particles are disclosed in U.S. Pat. No.3,971,373 to Braun, U.S. Pat. No. 4,429,001 to Kolpin et al. and U.S.Pat. No. 6,102,039 to Springett et al. Activated carbon and alumina areparticularly preferred sorbent particles. Mixtures of sorbent particlescan be employed, e.g., to absorb mixtures of gases, although in practiceto deal with mixtures of gases it may be better to fabricate amultilayer pleated filter employing separate sorbent particles in theindividual layers.

A variety of primary and secondary fluid streams may be employed to makethe disclosed filtration webs. Air is an especially convenient fluid forboth purposes. The remainder of this application will discuss the use ofair, sometimes referred to as “primary air” or as “secondary quench air”as the context may require. Those skilled in the art can readily employother fluids (e.g., carbon dioxide, nitrogen or water) with appropriatemodification of the operating parameters described below.

FIG. 1 shows a schematic side view of meltblowing apparatus 10. Moltenpolymer enters meltblowing die 12 through inlet 14 and passes throughdie cavity 16. Small orifices (not shown in FIG. 1) in die tip 18 causethe molten polymer to form filaments 22 upon exiting die 12. Primary airsupplied through inlets 20 impinges upon the filaments 22 and attenuatesthem into fibers 24. Fibers 24 land on flat collector 26 and formnonwoven web 28 which can be drawn away from collector 26 in thedirection of web formation (viz., the machine direction) 30 by asuitable take-up apparatus (not shown in FIG. 1). On route to collector26, secondary quench air supplied to ducts 32 arrayed across the widthof web 28 impinges upon the filaments or fibers, causing the fibers tooscillate to and fro generally in and against the machine direction. Thecollected fibers in the resulting web 28 are substantially more alignedin the machine direction than would be the case without the secondaryquench air supply. The web's machine direction and transverse directionmechanical properties (e.g., its machine direction and transversedirection stiffness and tensile strength) also exhibit greateranisotropy than when a secondary quench air supply is not employed.

Viewed from the side (or transverse direction) using high-speedphotography, fibers 24 are laid down on collector 26 in a “paintbrush”fashion. Measured at the collector, the oscillations can have a verylarge machine direction amplitude, e.g., more than one fourth the die tocollector distance (“DCD”) and in some instances more than half the DCD.Several operating conditions may be especially desirable to achieve suchpaintbrush deposition. For example, the oscillations may occurregularly, may have increasing amplitude en route to the collector, andmay have a wavelength for one complete cycle that is less than thedistance from the secondary quench air outlets to the collector.Preferably the distance from the secondary quench air outlets to thecollector is not overly long. The fibers may in some instances exhibit awhip-like action at their peak machine direction displacement en routeto the collector, and may momentarily move towards the meltblowing dierather than always moving toward the collector. Apparent fiber breakagecan sometimes be seen as such whip-like action occurs.

We have been able to tease fibers with discrete lengths (e.g., betweenabout 1 and about 10 cm, along with occasional shorter or longer fibers)from the collected webs using tweezers. Ordinarily it is quite difficultto remove any fibers (or any fibers of such lengths) from conventionalmeltblown webs, as the fibers typically are restrained in the web byfiber-to-fiber bonding or by interfiber entanglement.

Web 28 can be pleated as is, or further treated. Preferably a heattreatment (e.g., annealing) is employed to stiffen the web. Heattreatments may however make it more difficult to tease fibers from theweb, as the fibers may tend to fracture and the web may have greaterinter-fiber bonding or entanglement. Preferred annealing times andtemperatures will depend on various factors including the polymericfibers employed. As a general guide, annealing times and temperatures ofabout 100° C. up to the polymer melting point for a time less than about10 minutes are preferred.

A vacuum can optionally be drawn thorough orifice 34 to assist inconsolidating web 28. Overdensification (e.g., using calendaring) mayhowever destroy the web's filtration capability. Electric charge can beimparted to the fibers by contacting them with water as disclosed inU.S. Pat. No. 5,496,507 to Angadjivand et al., corona-treating asdisclosed in U.S. Pat. No. 4,588,537 to Klasse et al., hydrocharging asdisclosed, for example, in U.S. Pat. No. 5,908,598 to Rousseau et al. ortribocharging as disclosed in U.S. Pat. No. 4,798,850 to Brown.Additives may also be included in the fibers to enhance the web'sfiltration performance, mechanical properties, aging properties, surfaceproperties or other characteristics of interest. Representativeadditives include fillers, nucleating agents (e.g., MILLAD™ 3988dibenzylidene sorbitol, commercially available from Milliken Chemical),UV stabilizers (e.g., CHIMASSORB™ 944 hindered amine light stabilizer,commercially available from Ciba Specialty Chemicals), cure initiators,stiffening agents (e.g., poly(4-methyl-1-pentene)), surface activeagents and surface treatments (e.g., fluorine atom treatments to improvefiltration performance in an oily mist environment as described in U.S.Pat. Nos. 6,398,847 B1, 6,397,458 B1, and 6,409,806 B1 to Jones et al.).The types and amounts of such additives will be apparent to thoseskilled in the art.

The completed webs may have a variety of effective fiber diameter(“EFD”) sizes, basis weights and solidity (ratio of polymer volume toweb volume) values. Preferred EFDs are about 8 to about 25, morepreferably about 10 to about 25 μm. Preferred basis weights are about 50to about 100 g/m². Preferred solidity values are about 5 to about 15%.

The disclosed webs have substantial machine direction (direction ofmotion or direction of web formation) fiber alignment. As a generalguide for pleated filters from polypropylene webs, preferably about 55to about 90% of the fibers are aligned at 90°±20° with respect to therow direction, and more preferably about 70 to about 85%. For webs madefrom other polymeric materials the numbers may be lower or higher. Forexample, as a general guide for pleated filters made from polyethyleneterephthalate webs, preferably about 51 to about 80% of the fibers arealigned at 90°±20° with respect to the row direction, and morepreferably about 60 to about 80%. As a general guide for pleated filtersmade from nylon webs, preferably about 51 to about 70% of the fibers arealigned at 90°±20° with respect to the row direction. Very highlyaligned webs can be formed, e.g., webs having at least 80% of thecollected fibers aligned at 90°±20° with respect to the row direction.

The disclosed webs have one or more anisotropic mechanical properties.One class of preferred webs may have at least a 2:1 ratio of thein-plane tensile strength in the direction transverse to the rowdirection to the tensile strength in the row direction using a 50 mmgauge length, and more preferably at least a 3:1 ratio. Another class ofpreferred webs may have at least a 2:1 ratio of the in-plane TABERStiffness in the direction transverse to the row direction to the TABERStiffness in the row direction, and more preferably at least about2.2:1.

It is possible to construct the disclosed meltblowing apparatus andoperate it under conditions that do not provide the disclosed alignedfiber webs, or under conditions that will provide weak webs poorlysuited to filtration. For example, if insufficient secondary quench airis employed then the above-described oscillations may not occur and thefibers may not align substantially in the machine direction. Excessivelyhigh quench velocities may provide loftier webs having less interfiberbonding and entanglement and improved filtration performance, but havingseverely diminished mechanical properties such as stiffness andpleatability. Thus it generally will be preferable to employ secondaryquench air within a range of mass flow ratios or volumes. As an examplefor webs made using polypropylene and secondary air chilled to belowambient temperature, a ratio of about 800 to about 2000 grams ofsecondary quench air per gram of extruded polymer may be preferred, asmay be secondary quench air outlet velocities of about 15 to about 60m/sec. These ranges may need to be adjusted empirically based on factorssuch as the meltblowing die and polymer employed, the target basisweight, target web solidity and target extent of fiber alignment andmechanical property anisotropy. Pulsation of the secondary quench airmay also be employed but appears not to be necessary. Instead it appearsto be better simply to adjust the secondary quench air flow upwards ordownwards to a steady state value that provides collected webs havingthe desired final properties.

FIG. 2 shows a schematic overhead view of collector 26. Die 12 ispositioned close to the leading edge of collector 26, but can be moveddownweb to positions such as position 36 in order to alter theproperties of web 28. Such repositioning may for example provide webshaving reduced TABER Stiffness. If a conventional cylindrical collectorsurface is employed instead of a flat collector then it usually will bemore difficult to obtain webs whose fibers are substantially aligned inthe machine direction, and the webs may have lower TABER Stiffness.Excessive DCD lengths or excessive distances from the secondary quenchair outlets to the collector may also be detrimental, e.g., by causingtoo many oscillations en route to the collector, excessive fiberattenuation or excessive fiber breakage.

Further details regarding the manner in which meltblowing would becarried out using an apparatus like that shown in FIG. 1 and FIG. 2 willbe familiar to those skilled in the art.

The nonwoven filtration media can be further stiffened if desired usinga variety of techniques. For example, an adhesive can be employed tolaminate together layers of the filter media, e.g., as described in U.S.Pat. No. 5,240,479 (Bachinski). The filter media can also be made usingconjugate fibers, e.g., as described in U.S. Pat. No. 5,709,735 (Midkiffet al.). Further details regarding the nonwoven filtration media canalso be found in copending U.S. patent application Ser. No. 10/818,096(now U.S. Pat. No. 6,858,297 B1), filed Apr. 5, 2004 and entitledALIGNED FIBER WEB, the disclosure of which is incorporated herein byreference.

FIG. 7 shows pleated filter media 100 having rows of pleats 102. Therows are aligned in the transverse direction, and thesubstantially-aligned fibers in web 100 are aligned at 90°±20° withrespect to the row direction, that is, at ±20° with respect to themachine direction. FIG. 8 shows pleated filter media 100 and an expandedmetal support 110 mounted in frame 112 to provide filter 114. Theincreased stiffness of pleated media 100 and the substantial machinedirection fiber alignment transverse to the row direction both arebelieved to contribute to increased resistance of pleated media 100 topleat deformation at high filter face velocities.

FIG. 10 shows an apparatus 120 for making the disclosed pleated filters.Aligned filtration media 122 is typically provided on a continuous roll124. Media 122 may be slit to a desired width at slitting station 126.Media 122 may optionally be preheated at a heating station 128 toperform annealing or to make web 122 more flexible while passing into orthrough apparatus 120. In the illustrated embodiment, the heatingstation 128 is an infrared heater.

In one pleated filter embodiment, a reinforcing member 130 is applied toa rear face 132 of media 122 at location 134. Reference to the rear face132 (or to the front face 164, discussed below) is for purposes ofdescription only and does not indicate a required airflow orientation ofthe completed pleated filter. For example, the reinforcing member 130may be positioned upstream or downstream in the air flow. Reinforcingmember 130 may be applied in a variety of positions, e.g., as one ormore continuous strips oriented in the machine direction 136, asdiscrete reinforcing members oriented transverse to the machinedirection 136, or in other configurations that will be familiar to thoseskilled in the art. Reinforcing member 130 may be bonded to media 122using a variety of techniques, such as adhesive bonding, thermalbonding, solvent bonding, or ultrasonic bonding. In this embodiment,location 134 is upstream from rotary-score pleater 138 which scoresmedia 122 and reinforcing member 130 prior to pleating at pleat foldingstation 140. Infrared heaters 142 may optionally be provided forheat-setting the pleats 144. The pleats 144 are retained in anaccumulator 146 and then advanced to a pleat spacing device 148 thatretains the pleats 144 in the desired pleat spacing. Pleat formation andpleat spacing may be performed by a variety of methods, such asdisclosed in U.S. Pat. Nos. 4,798,575 (Siversson '575), 4,976,677(Siversson '677) and 5,389,175 (Wenz).

The resulting pleated filter media 100 is expanded to the desired pleatspacing in the pleat spacing device 148. One or more elongated, planarreinforcing strips 162 may optionally be applied to the pleat tips alongthe filter front face 164 at station 168 to maintain the spaces betweenthe spaced-apart pleats. The reinforcing strips 162 may be bonded to thepleat tips by various techniques, such as adhesive bonding, thermalbonding, solvent bonding, or ultrasonic bonding, and can provideadditional dimensional stability to the pleats 144. The pleated filtermedia 100 and optional reinforcing strip 162 can be cut to a desiredsize and shape. Pleated filter media 100 may be used in filtrationapplications, with or without a frame structure, or as an insert into apermanent or a reusable frame.

In another embodiment, a scrim 162 extending substantially across theentire front face 164 may be employed. Scrim 162 may be bonded to thepleat tips to provide additional dimensional stability to pleated filtermedia 100.

In yet another embodiment, one or more elongated, planar reinforcingstrips 166 may optionally be bonded to the rear face 132 of pleatedfilter media 100 at station 168. In a further embodiment, thereinforcing strips 166 may be located over the reinforcing member 130and opposite the reinforcing strips 162 to form truss structures asshown in FIG. 6 of Sundet et al. '011.

In embodiments where the pleated filter media 100 is formed without asurrounding frame, the pleated filter 172 exits the system 120 after thecutting station 186.

In a framed filter embodiment, a continuous strip of frame material 180may be applied to the side edges of pleated filter media 100 parallel tothe machine direction 136. An adhesive, such as a hot melt adhesive, maybe applied to a first flange of a U-shaped channel formed from framematerial 180 at station 187. An adhesive for sealing the ends of thepleats 144 may be applied at station 188. An adhesive may be applied toa second flange of the U-shaped channel at station 190. The framematerial 180 may be bent into a U-shaped configuration at station 194.The ends of U-shaped channel may extend partially onto the front face164 and rear face 132 of the pleated filter media 100. An assemblyincluding the endless web of pleated filter media 100 and attached framematerial 180 may be cut at station 186 to desired lengths. The pleatedfilter media 100 and reinforcing strips 162 may also be cut to sizebefore application of frame material 180.

The pleated filter media 100 and side frames members 192 may be rotated90° at location 191 to permit application of end frame members 196 atstation 198 and formation of framed pleated filter 174. Framed pleatedfilter 174 may also be formed by configuring members 192, 196 as two boxstructures that are positioned over the first and second faces 132, 164,respectively, of pleated filter media 100, with overlappedcircumferential portions, such as disclosed in U.S. Pat. No. 5,782,944(Justice). In another embodiment, the frame member members 192, 196 canbe configured as a one-sided die-cut frame that is folded around framedpleated filter 174.

The pleated filters 172, 174 are typically enclosed in suitablepackaging. For typical HVAC applications, there typically are about 3 toabout 6 pleats per 25.4 centimeters (1 inch). The pleat depth and filterthickness is typically about 25 centimeters to about 102 centimeters (1inch to 4 inches). The filter length and width is typically about 30.5centimeters×30.5 centimeters (12 inches×12 inches) to about 50.8centimeters×122 centimeters (20 inches×48 inches).

Further details regarding pleated filter manufacture can be found in theabove-mentioned Sundet et al. '011 and Sundet et al. '100 patents orwill be familiar to those skilled in the art.

The disclosed pleated filters may be employed in a variety ofapplications including ventilation (e.g., furnace and clean roomfilters), pollution control (e.g., baghouse filters), liquid treatment(e.g., water filters), personal protection (e.g., protective suits withpowered air supplies) and other applications that will be familiar tothose skilled in the art.

The invention will now be described with reference to the followingnon-limiting examples, in which all parts and percentages are by weightunless otherwise indicated. Several measurements were carried out asfollows:

Effective Fiber Diameter

Effective geometric fiber diameters were evaluated according to themethod set forth in Davies, C. N., “The Separation of Airborne Dust andParticles,” Institution of Mechanical Engineers, London, Proceedings 1B,1952.

Optical/Visual Web Properties, Fiber Counts and Fiber Alignment

The overall visual web appearance was evaluated using a ZeissInstruments dissecting microscope equipped with a charge coupled devicecamera having an 8 mm×14 mm magnification window. Fiber count and fiberalignment data were obtained using one or two swatches cut from themiddle of the web. The swatches were labeled on each side and themachine direction was noted. Microscopic examination was used to countfibers and determine fiber alignment, and the resulting values for eachside (and where two swatches were obtained, each swatch) were averagedtogether.

TABER Stiffness

Web stiffness was evaluated using a Model 150-B TABER™ stiffness tester(commercially available from Taber Industries). Square 3.8 cm×3.8 cmsections were carefully vivisected from the webs using a sharp razorblade to prevent fiber fusion, and evaluated to determine theirstiffness in the machine and transverse directions using 3 to 4 samplesand a 15° sample deflection.

Stress-Strain

Stress-strain (or load vs. elongation) was measured using a Model 5544INSTRON™ universal testing machine (commercially available from InstronCorp.). Rectangular 2.5 cm×6.3 cm sections were cut from the webs usinga sharp razor blade and evaluated to determine the maximum force andelongation at maximum force, using 6 to 10 samples, a 50 mm initial jawseparation and a 3 cm/min stretch rate.

Filtration Quality Factor

Filtration quality factors (Q_(F)) were determined using a TSI™ Model8130 high-speed automated filter tester (commercially available from TSIInc.) and a dioctyl phthalate (“DOP”) challenge aerosol flowing at 42.5L/min. Calibrated photometers were employed at the filter inlet andoutlet to measure the DOP concentration and the % DOP penetrationthrough the filter. An MKS pressure transducer (commercially availablefrom MKS instruments) was employed to measure pressure drop (ΔP, mm H₂O)through the filter. The equation:

$Q_{F} = \frac{- {\ln\left( \frac{\%\mspace{14mu}{DOP}\mspace{14mu}{penetration}}{100} \right)}}{\Delta\; P}$was used to calculate Q_(F). Q_(F) values can be reported as a curveplotting Q_(F) vs. the DOP challenge total mass after various timeperiods. However, the initial Q_(F) value usually provides a reliableindicator of overall performance, with higher initial Q_(F) valuesindicating better filtration performance and lower initial Q_(F) valuesindicating reduced filtration performance. Initial filtration qualityfactors Q_(F) of at least about 0.6 (using 100 ppm dioctyl phthalateparticles having a size range between 10 and 700 nm traveling at a 7cm/sec face velocity), more preferably at least about 0.8 and mostpreferably at least about 1 are preferred.

Filtration Performance

Filtration performance was evaluated according to ASHRAE standard 52.2,“Method of Testing General Ventilation Air-Cleaning Devices for RemovalEfficiency by Particle Size”. The ASHRAE standard evaluates filtrationof a test aerosol containing laboratory-generated potassium chlorideparticles dispersed into an airstream. A particle counter measures andcounts the particles in 12 size ranges upstream and downstream from thefilter. The results can be reported as minimum composite efficiencyvalues for particles in various size ranges. The minimum compositeefficiency values correspond to the minimum percent particle retention(the downstream particle count/upstream particle count ×100 for the sizerange in question) as the filter is loaded to a final pressure drop of25.4 mm H₂O. A set of particle size removal efficiency performancecurves at incremental dust loading levels may also be developed, andtogether with an initial clean performance curve may be used to form acomposite curve representing the minimum performance in each size range.Points on the composite curve are averaged and the averages used todetermine the minimum efficiency reporting value for the filter.

EXAMPLES 1-2 AND COMPARISON EXAMPLES 1-2 Polypropylene Webs

A conventional 20.5 cm wide meltblowing apparatus was modified byaddition of a secondary air quench system and a flat-bed collectorarranged as in FIG. 1. In a conventional blown microfiber process,secondary quench air would not be employed and the web would becollected on a rounded surface such as a porous drum. The modifiedapparatus was used to make meltblown polypropylene webs whose fiberswere highly aligned in the machine direction. The secondary air quenchsystem employed two opposed horizontally-disposed 76 cm wide×51 cm highair outlets disposed approximately 6 cm below the meltblowing die tip,dispensing 12-13° C. chilled air flowing at various rates (or not atall) through the air outlets. The flat-bed collector employed a vacuumcollection system located under the bed. The meltblowing die waspositioned over the leading edge of the collector. FINA™ type 3960polypropylene (commercially available from Fina Oil and Chemical Co.)was melted in an extruder operated at 265° C. and fed at 9.1 kg/hr tothe meltblowing die. The die was maintained at about 265° C. usingresistance heaters and supplied with 300° C. primary air flowing at 4.2m³/min. The DCD was adjusted to provide webs having a 0.5 mm H₂Opressure drop at a 32.5 L/min flow rate. For webs prepared usingsecondary quench air, the DCD was approximately 20 cm. For webs preparedwithout secondary quench air, the DCD was approximately 34 cm. Thecollector vacuum was adjusted to provide webs having 8-9% solidity. Thecollector vacuum was 3250 N/m² for webs prepared using secondary quenchair at 50 or 35 m/sec outlet velocity, 5000 N/m² for webs prepared usingsecondary quench air at 17 m/sec outlet velocity, and zero for websprepared without secondary quench air. The collected webs had an 80 g/m²basis weight and a 19 μm EFD. The webs were corona-treated as describedin U.S. Pat. No. 4,588,537 to Klasse et al., hydrocharged as describedin U.S. Pat. No. 5,908,598 to Rousseau et al. and evaluated to determinetheir mechanical properties and filtration quality factor Q_(F). Thewebs were also heat treated at 126° C. for 5 minutes and reevaluated todetermine their mechanical properties.

Set out below in Table 1 are the Example No. or Comparison Example No.,secondary air velocity and mass flow ratio, the fiber count and fiberalignment data, and the filtration quality factor Q_(F) for each web.One swatch from the middle of each web was obtained and examined on bothsides (viz., the collector and non-collector sides), the fibers andtheir alignments were noted and the results for both sides were averagedtogether. Set out below in Table 2 are the machine direction (“MD”) andtransverse direction (“TD”) TABER Stiffness and tensile strength valuesfor the each web, and the ratio of MD to TD TABER Stiffness and tensilestrength. Set out below in Table 3 are the MD and TD TABER Stiffness andtensile strength values for the heat treated webs, and the ratio of MDto TD TABER Stiffness and tensile strength.

TABLE 1 Fibers Example Secondary No. of Fibers No. of Within No. or AirMass Total No. Fibers Within Fibers 0-20° Filtration Comp VelocityRatio, of Within 0-10° Within Of Quality Example at Outlet SecondaryMeasured 0-10° of MD 0-20° MD Factor, No. (m/sec) Air:Polymer Fibers ofMD (%) of MD (%) Q_(F) 1 50 1770 119 59 50 83 70 1.65 2 35 1450 145 6343 88 61 1.5 Comp. 17 720 200 49 25 87 44 0.85 Ex. 1 Comp. 0 0 200 60 3085 43 0.7 Ex. 2

TABLE 2 Example Secondary No. or Air Mass Ratio, Tensile Tensile Ratio,Comp. Velocity Ratio, TABER TABER TABER Strength, Strength, TensileExample at Outlet Secondary Stiffness, Stiffness, Stiffness MD TDStrength No. (m/sec) Air:Polymer MD TD MD:TD (dynes) (dynes) MD:TD 1 501770 1.3 0.6 2.2 1773 635 2.8 2 35 1450 1.9 0.7 2.7 2270 730 3.1 Comp.Ex. 1 17 720 2.1 1.1 1.9 1374 1101 1.2 Comp. Ex. 2 0 0 1.5 1.3 1.2 16701370 1.2

TABLE 3 (Heat Treated) Example Secondary No. or Air Mass Ratio, TensileTensile Ratio, Comp. Velocity Ratio, TABER TABER TABER Strength,Strength, Tensile Example at Outlet Secondary Stiffness, Stiffness,Stiffness MD TD Strength No. (m/sec) Air:Polymer MD TD MD:TD (dynes)(dynes) MD:TD 1 50 1770 1.2 0.7 1.7 2298 780 2.9 2 35 1450 1.9 0.7 2.72303 765 3.0 Comp. Ex. 1 17 720 2.5 1.6 1.6 1770 1100 1.6 Comp. Ex. 2 00 2.5 2.0 1.3 1700 1489 1.1

As shown in Table 1, webs made using sufficient secondary quench air hadsignificantly greater machine direction fiber alignment than a web madewithout secondary quench air or a web made with insufficient secondaryquench air. This is further illustrated in FIGS. 3 a-6 b, which arepolar “radar” plots for samples taken respectively from the collectorside (see FIG. 3 a, FIG. 4 a, FIG. 5 a and FIG. 6 a) and from thenon-collector side (see FIG. 3 b, FIG. 4 b, FIG. 5 b and FIG. 6 b) ofthe webs in Examples 1-2 and Comparison Examples 1-2. The plots show thenumber and orientation (in degrees with respect to the 0° machinedirection) of fibers in each sample. Because fibers oriented at 0° withrespect to the machine direction could also be said to be oriented at180°, the plots have symmetric lobes reflected about the origin. FIG. 4a shows for example that 12 fibers were oriented at 0° with respect tothe machine direction, 9 fibers were oriented at −10°, 8 fibers wereoriented at +10°, 6 fibers were oriented at −20°, and so on. FIGS. 3 a-4b show that the disclosed webs had considerably greater machinedirection alignment than the webs plotted in FIG. 5 a-FIG. 6 b. In afurther comparison, the web shown in FIG. 4 of the Lau et al. '749patent was evaluated to determine its fiber orientation and fiber count,and found to have only 50% of its fibers aligned within ±20° of themachine direction.

The FIG. 3 a-4 b plots generally mirror the behavior of a wetting fluidplaced on the non-collector side of the web. If a drop of a suitablewetting fluid (preferably colored to aid in observation) is so placed itwill tend to spread into the web in an oblong pattern generallycorresponding to the radar plot lobe shapes, thus providing a convenientindicator of the primary direction of fiber orientation and webformation. When a wetting fluid is placed on the Comparison Example 2web, it tends to spread more or less evenly outward in an expandingcircular pattern.

Table 1 also shows that as the secondary quench air volume increased,the filtration quality factor Q_(F) increased.

Webs prepared using sufficient secondary air had visible striationsgenerally aligned in the machine direction, a surface with an overallsmooth sheen and slight fuzziness, and few or none of the nodules thatusually are found in conventional blown microfiber webs. Fibers havingapproximately 2 to 5 cm lengths could be teased from the webs usingtweezers. Webs prepared with insufficient secondary quench air visuallyresembled standard blown microfiber webs collected on a round collector.Some relatively short (less than 1 cm) individual fibers could beremoved from these webs using tweezers, but only with great difficulty.

As shown in Table 2, webs made using sufficient secondary quench air hadmore anisotropic TABER Stiffness than webs made with insufficientsecondary quench air. Webs made using sufficient secondary quench airalso had significantly more anisotropic tensile strength than webs madewith insufficient secondary quench air.

As shown in Table 3, heat treating could be used to increase webstiffness and tensile strength. For the Example 1 and Example 2 websthis was done without causing a substantial change in the web's overallmechanical anisotropy as measured using MD:TD property ratios.

EXAMPLES 3-5 Smaller EFD Webs

Using the general method of Example 1, meltblown polypropylene webs wereprepared using 300° C. primary air flowing at 3.4 m³/min and secondaryquench air and collector vacuum adjusted to provide collected webshaving an 80 g/m² basis weight, 8-9% solidity and smaller effectivefiber diameters than were obtained in Example 1. One swatch from themiddle of each web was obtained and examined on each side. Set out belowin Table 4 are the Example No., secondary air velocity and mass flowratio, effective fiber diameter and the fiber count and fiber alignmentdata for the resulting webs.

TABLE 4 Fibers Fibers Secondary No. of within No. of Within Air TotalNo. Fibers 0-10° Fibers 0-20° Velocity Mass Ratio, of Within of Withinof Example at Outlet Secondary Measured 0-10° MD 0-20° MD No. (m/sec)Air:Polymer EFD Fibers of MD (%) of MD (%) 3 22.5 930 13.5 120 53 44 7462 4 23 950 10 120 48 40 68 57 5 14.5 620 7.4 120 58 48 81 68

As shown in Table 4, webs having substantial machine direction fiberalignment could be made at a variety of effective fiber diameters.

EXAMPLES 6-8 AND COMPARISON EXAMPLES 3-5 PET and Nylon Webs

Using the general method of Example 1, polyethylene terephthalate(“PET”) and nylon (ULTRAMID™ BS-400N nylon, commercially available fromBASF Corp.), were employed to prepare nonwoven webs using 350° C.primary air flowing at 2.9 m³/min and optional secondary quench air. A12.7 cm DCD was used to prepare the PET webs and a 16.5 cm DCD was usedto prepare the nylon webs. The collected PET webs had an 85 g/m² basisweight, 5-6% solidity and a 16 μm EFD. The collected nylon webs had a 70g/m² basis weight, 5-6% solidity and a 17-18 μm EFD. Two swatches wereobtained and examined on each side for the Example 8 web, and one swatchwas obtained and examined on each side for the remaining webs. Set outbelow in Table 5 are the Example No. or Comparison Example No.,secondary air velocity and mass flow ratio, polymer employed and thefiber count and fiber alignment data for the resulting webs.

TABLE 5 No. of Fibers No. Fibers Secondary Fibers within of Fiberswithin Example Air Total within 0-10° within 0-20° No. or Velocity MassRatio, No. of 0-10° Of 0-20° of Comp. at Outlet Secondary Measured of MDof MD Ex. No (m/sec) Air:Polymer Polymer Fibers MD (%) MD (%) 6 35 1450PET 120 51 43 73 61 7 22.5 930 PET 120 44 37 69 58 Comp. 0 0 PET 120 2118 32 27 Ex. 3 8 35 1450 Nylon 400 105 26 216 54 Comp. 22.5 930 Nylon120 45 38 56 47 Ex. 4 Comp. 0 0 Nylon 120 33 28 53 44 Ex. 5

As shown in Table 5, webs having substantial machine direction fiberalignment could be made using a variety of polymers.

EXAMPLE 9 Additive

Example 2 was repeated using a 1.5% addition of the additivepoly(4-methyl-1-pentene). This increased the filtration quality factorQ_(F) from 1.5 without the additive to 1.7 with the additive.

EXAMPLE 10 Additives

Example 9 was repeated using a 1.5% addition of the additivepoly(4-methyl-1-pentene) and a 0.5% addition of CHIMASSORB 944 hinderedamine light stabilizer. The web was hydrocharged but not corona-treated.The filtration quality factor Q_(F) was 2.5, and more than double thatobtained using webs made from conventional untreated polypropylene blownmicrofibers made without secondary quench air.

EXAMPLE 11 AND COMPARISON EXAMPLE 6 Q_(F) Evaluation Over Time

Using the general method of Example 1, meltblown polypropylene webs wereprepared with and without secondary quench air flowing at a 1770secondary quench air:polymer mass flow ratio, corona-treated,hydrocharged, and evaluated to determine their filtration quality factorQ_(F). The webs had an 85 g/m² basis weight, 19-21 μm EFD, and apressure drop of 0.4-0.5 mm H₂O at 42.5 L/min. Set out below in Table 6are the Q_(F) factors after various cumulative DOP exposure levels forthe web made with (Example 11) or without (Comparison Example 6)secondary quench air.

TABLE 6 Cumulative DOP Example Comparison Challenge, 11 Example 6 (mg)Web, Q_(F) Web, Q_(F) Initial 1.8 0.7  50 1 0.2 100 0.6 0.15 150 0.450.1 200 0.3 <0.1

As shown in Table 6, a web made using secondary quench air and havingsubstantial machine direction fiber alignment provided significantlybetter filtration performance than a web made without secondary quenchair and having less fiber alignment.

EXAMPLES 12-14 AND COMPARISON EXAMPLES 7-9 Pleated Furnace Filters

The Example 2 and Example 10 webs and the Comparison Example 2 web werecorona-treated, hydrocharged, or both corona-treated and hydrocharged. Asample of ACCUAIR™ spunbond polyethylene/polypropylene twinned fiber web(71 g/m² basis weight, ˜20 μm EFD, commercially available from KimberlyClark Corp. and corona-treated as supplied) was also obtained. Thetreated webs were formed into 50.8 cm×63.5 cm×2.1 cm high filterelements having spaced-apart pleats 102 like pleated media 100 shown inFIG. 7. The pleats 102 were arranged so that the folds were aligned withthe transverse direction, with 87 pleats (13.8 pleats/10 cm) along thelong dimension. The pleated media 100 was sandwiched between and gluedto expanded metal supports like support 110 and mounted in a cardboardframe like frame 112 shown in FIG. 8 to form a framed filter like filter114. The finished filters were evaluated for filtration performanceaccording to ASHRAE standard 52.2, “Method of Testing GeneralVentilation Air-Cleaning Devices for Removal Efficiency by ParticleSize” at a 1.5 m/sec face velocity. The results reported below in Table7 show the minimum composite efficiency values for particles in the sizeranges 0.3 to 1 μm, 1 to 3 μm and 3 to 10 μm. Table 7 also reports thetotal filter weight gain (total particulate weight captured by thefilter) after completion of the evaluation. Unless otherwise indicated,the weight gain values were obtained at a pressure drop corresponding to25.4 mm H₂O. Higher minimum composite efficiency and total filter weightgain values correspond to better filtration, longer service life or bothbetter filtration and longer service life.

TABLE 7 Total Filter Minimum Weight Ex. No. Initial Composite Gain, orPressure Efficiency (%) 25.4 mm Comp. Web Web Drop, 0.3 1 to 3 to H₂OEx. No. Employed Treatment mm H₂O to 1 μm 3 μm 10 μm (g) 12 Example 2Corona- 4.9 43 81 91 35 treated 13 Example 2 Corona- 5.1 53 89 97 37.1treated and hydrocharged 14 Example 10 Hydrocharged 5.1 56 90 98 40.4(at 35.6 mm H₂O) Comp. Comp. Ex. 2 Corona- 4.6 36 71 86 23.8 Ex. 7treated Comp. Comp. Ex. 2 Corona- 4.6 42 81 93 33.9 Ex. 8 treated andhydrocharged Comp. ACCUAIR Corona- 4.6 41 80 91 26.1 Ex. 9 web treated

As shown in Table 7, corona-treated, hydrocharged, or corona-treated andhydrocharged meltblown nonwoven webs made using secondary quench air andhaving substantial machine direction fiber alignment (Example 12 throughExample 14) provided much better minimum composite efficiency thanotherwise similar meltblown nonwoven webs made without secondary quenchair and having less fiber alignment (Comparison Examples 7 and 8). TheExample 12 through Example 14 webs also had comparable or better minimumcomposite efficiency than a commercial spunbond nonwoven web (ComparisonExample 9). The Example 12 and Example 13 webs had better holdingcapacity (as evidenced by their higher total filter weight gain values)at a 25.4 mm H₂O pressure drop) than the Comparison Example 7-9 webs.

EXAMPLES 15-17 AND COMPARISON EXAMPLE 10 Pleated Furnace Filter PressureProp Vs. Face Velocity Evaluation

Using the general method of Examples 12 and 13, a meltblown alignedfiber polypropylene web was corona-treated and hydrocharged but not heattreated. This web had a 1.7 MD TABER Stiffness value and is identifiedbelow as the web of Example 15. A stiffer web was prepared usingcorona-treatment, hydrocharging and heat treatment. This web had a 2.2MD TABER Stiffness value and is identified below as the web of Example16. A yet stiffer web was prepared using a 1.5% addition of the additivepoly(4-methyl-1-pentene), corona-treatment, hydrocharging and heattreatment. This web had a 3.7 MD TABER Stiffness value and is identifiedbelow as the web of Example 17. The Example 15-17 webs and a sample ofACCUAIR corona-treated spunbond polyethylene/polypropylene twinned fiberweb (having a 2.1 MD TABER Stiffness and identified below as the web ofComparison Example 8) were formed into 30 cm×27 cm×2.1 cm high filterelements having spaced-apart pleats 102 like filter media 100 shown inFIG. 7. The filters had 13.8 pleats/10 cm along the long dimension, andwere sandwiched between and glued to expanded metal supports likesupport 110 in FIG. 8. In a series of runs, each such filter was mountedin a PLEXIGLAS™ plastic frame whose transparent side plates permittedthe pleat edges to be photographed. The frame side plates touched thefiltration media edges but permitted pleat movement. The frame wasmounted atop a vacuum table and exposed to air from adownwardly-directed box fan. The filters were loaded by sprinkling asynthetic dust made from a 50:50 mixture of SAE Fine Test Dust and talcinto the air stream until the filter pressure drop reached about 0.35 in(0.9 cm) of water at an approximate 1.5 msec face velocity. Thissimulated a substantial natural loading level. Set out below in Table 8are the filter descriptions, MD TABER Stiffness values and total filterweight gain values.

TABLE 8 Example No. MD Total Filter Or TABER Weight Gain Comp. Ex. No.Stiffness (g) 15 1.7 37.9 16 2.2 40.2 17 3.7 33.4 Comp. Ex. 10 2.1 36.2

The filters were next mounted in a duct equipped with an anemometer andexposed to flowing air at velocities sufficient to cause pressure dropsbetween about 0.2 in. (0.5 cm) of water and 1.2 in. (3 cm) of water. TheExample 15 (1.7 MD TABER Stiffness) filter began to exhibit noticeablepleat deformation, manifested by pinching together of the pleats at thefilter air inlet side, at a 0.35 in (0.9 cm) pressure drop. The Example16 (2.2 MD TABER Stiffness) and Comparison Example 10 filters began toexhibit noticeable pleat deformation at a 0.5 in (1.3 cm) pressure drop.The Example 17 (3.7 MD TABER Stiffness) filter did not exhibitnoticeable pleat deformation even at a 1.2 in (3 cm) pressure drop. FIG.9 shows a plot of the pressure drop (in inches of water) vs. anemometerreading (in nominal units) for the filters of Example 16 (curve 116),Example 16 (curve 117), Example 17 (curve 118) and Comparison Example 10(curve 119). As shown in FIG. 9, the Example 17 filter exhibited alinear increase in pressure drop as airflow increased, whereas the othertested filters exhibited a non-linear response, (indicative of pleatdistortion and eventual loss of the space between pleats) as airflowincreased.

Various modifications and alterations of this invention will be apparentto those skilled in the art without departing from this invention. Thisinvention should not be restricted to that which has been set forthherein only for illustrative purposes.

1. A self-supporting pleated filter element comprised of a nonwovencontinuous filtration web having rows of folded or corrugatedspaced-apart, oppositely-facing pleats and that contains thermoplasticfibers a majority of which are individually aligned at 90°±20° withrespect to the tow direction; wherein the filtration web exhibits aneffective fiber diameter of about 8 to about 25 μm.
 2. A filter elementaccording to claim 1 wherein about 55 to about 90% of the fibers arealigned at 90°±20° with respect to the row direction.
 3. A filterelement according to claim 1 wherein about 70 to about 85% of thecollected fibers are aligned at 90°±20° with respect to the rowdirection.
 4. A filter element according to claim 1 wherein individualfibers having lengths of about 2-5 cm can be teased from the web.
 5. Afilter element according to claim 1 wherein the web has at least a 2:1ratio of the in-plane tensile strength in the direction transverse tothe row direction to the tensile strength in the row direction using a50 mm gauge length.
 6. A filter element according to claim 1 wherein theweb has at least a 4:1 ratio of the in-plane tensile strength in thedirection transverse to the row direction to the tensile strength in therow direction using a 50 mm gauge length.
 7. A filter element accordingto claim 1 wherein the web has at least a 2:1 ratio of the in-planeTaber Stiffness in the direction transverse to the row direction to theTaber Stiffness in the row direction.
 8. A filter element according toclaim 1 wherein the web has at least a 2.2:1 ratio of the in-plane TaberStiffness in the direction transverse to the row direction to the TaberStiffness in the row direction.
 9. A filter element according to claim 1wherein the major surfaces of the web exhibit striations correspondingto substantial alignment of individual fibers transverse to the rowdirection.
 10. A filter element according to claim 1 wherein a wettingfluid placed on the web preferentially wicks transverse to the rowdirection.
 11. A filter element according to claim 1 wherein the web hasbeen annealed.
 12. A filter element according to claim 1 wherein the webhas been corona-treated or hydrocharged.
 13. A filter element accordingto claim 1 wherein the filter element is an air filtration element andwherein the web has a filtration quality factor Q_(F) of at least about1.0 using 100 ppm dioctyl phthalate particles having a size rangebetween 10 and 700 nm traveling at a 7 cm/sec face velocity.
 14. An airfilter element according to claim 13 wherein the pleated filtration webis mounted in a frame.
 15. An air filter element according to claim 14wherein the frame comprises side frame members and end frame members,each frame member comprising a U-shaped channel.
 16. A filter elementaccording to claim 1 wherein the filter element does not include anelongated, planar reinforcing strip bonded to a face of the pleatedfiltration web.
 17. A filter element according to claim 1 wherein thepleats of the pleated filter have sufficient stiffness so that they donot collapse when subjected to the air pressure typically encountered inforced air ventilation systems.
 18. A filter element according to claim1 wherein the spaced-apart pleats are configured so that major surfacesof the web on adjacent pleats are not in face-to-face contact with eachother.
 19. A filter element according to claim 1 wherein the nonwovenweb comprises a meltblown web and wherein the thermoplastic fibers aremeltblown fibers.